1. Field of the Invention
The present disclosure relates to an electromagnetic switch for interrupting a current flowing through an electrical circuit by opening and closing an electrical contact.
2. Description of Related Art
Japanese Patent Application Laid-open No. 2003-297207 discloses an electromagnetic switch as shown in FIGS. 3A to 3C. This electromagnetic switch 100 includes an excitation coil 101, a movable core 102 driven by magnetic force generated by the excitation coil 101, and an electrical contact opened and closed in accordance with movement of the movable core 102. The electrical contact is constituted of a pair of fixed contacts 103 and a movable contact 104 movable in accordance with movement of the movable core 102.
The movable contact 104 is provided movably to be in contact and out of contact with the end portions on one axial end side of the fixed contacts 103. A shaft 107 is fixed to the movable core 102 so as to be abuttable against the end surface on the other axial end side of the movable contact 104. The fixed contacts 103 are held within a resin-made housing 108 accommodating a contact pressure spring 110 for biasing the movable contact 104 toward the other axial end side (toward the fixed contacts 103).
While the excitation coil 101 is deenergized, since the movable core 102 is biased toward the one axial end side by a return spring 111 causing the shaft 107 to push the movable contact 104 toward the one axial end side, the movable contact 104 is in the opened state in which the movable contact 104 is away from the fixed contacts 103. Movement of the movable contact 104 toward the one axial end side is restrained when the movable contact 104 abuts against a contact abutment portion 108a provided in the housing 108. When the excitation coil 101 is energized in this state, the movable core 102 is attracted toward the other axial end side against the biasing force of the return spring 111, causing the shaft 107 to move away from the movable contact 104. As a result, since the force pressing the movable contact 104 toward the one axial side is removed, the movable contact 104 is biased toward the fixed contacts 103 by the contact pressure spring 110 to be in the closed state in which it is in contact with the fixed contacts 103. The above described electromagnetic switch 100 is configured such that there is a clearance c between the movable core 102 and the fixed contacts 103 so that they do not contact with each other in the open state. However, abrasion may occur in the surface of the movable contact 104, the surface of the shaft 107 abutting against the movable contact 104, or the surface of the contact abutment portion 108a abutting against the movable contact 104 due to a larger number of times of operations or vibration of the electromagnetic switch 100. Such abrasion causes the clearance c to become smaller, as a result of which the fixed contacts 103 may be short-circuited with each other through the movable core 102 when the electromagnetic switch 100 in in the open state. To avoid this problem, it might occur that the clearance c is set larger in the initial state where there is no such abrasion. However, to do so, since it is necessary to increase the dimension of the electromagnetic switch 100 in the axial direction, the electromagnetic switch 100 has to be upsized.